KAPPA UMA (Kappa Ursae Majoris). With Talitha (Iota Ursae Majoris), Kappa makes
the eastern paw of Ursa Major, the Greater
Bear, and from ancient Arabic times the "third leap" of the Gazelle
(two other similar pairs
lying to the east). Such pairs can be
deceiving. While seemingly close to Talitha, at a distance of 425
light years Kappa is nearly nine times farther away. A white class
A (A1) star of magnitude four (3.60), it seemingly shines with the
light of 540 Suns from a surface heated to a temperature of 9600
Kelvin. Kappa, however, is deceptive, as it is not one star but
two nearly identical stars lying very close together, the
separation typically a mere couple tenths of a second of arc, the
brighter only 0.2 magnitudes above the fainter. Upon separation we
see that Kappa-A shines with the light of 290 Suns, Kappa-B with 250. Though the stars are
close, sophisticated optical systems have allowed the calculation
of an orbit. The two waltz around each other with a period of 35.6
years at an average separation of 24 Astronomical Units. A fairly
high eccentricity brings then as close as 11 AU, and as far apart
as 37 AU. From these parameters, we find a total mass to the
system of 11 times that of the Sun. However, temperature,
luminosity, and theory suggest masses of 3.4 solar for each of them
(for a total of 7 solar), showing that at least some of the
measures (distance, orbital period, and separation) are a bit off.
Both stars are close to giving up on core hydrogen fusion and are
turning into "subgiants," both having begun life as class B7 stars
some 250 million years ago. What makes the system more intriguing
is that one, or both, are rare class A "emission" (Ae) stars that
radiate energy from hydrogen, which is characteristic of a
circumstellar disk. Such stars are cooler counterparts of the much
better known B-emission (Be) stars, epitomized by Gamma Cas and Zeta
Tauri (which they might well have been). Consistently, one or
both rotate at a projected equatorial velocity of a quite-high 201
kilometers per second, which (allowing for axial tilt) gives a
rotation period of only 3.5 days (as opposed to 25 days for the
Sun), the rapid spin somehow connected to the orbiting
circumstellar disk.